When they form, do stars "know" what kind of galaxy they will inhabit? Obviously that question is too anthropomorphic—stars have no minds. Nevertheless, fundamental differences appear to exist in the populations of stars in spiral galaxies (like the Milky Way) and those in elliptical galaxies. Not only that, different elliptical galaxies also differ from each other, as determined by the motion of stars and the ratio of mass to light, which is a measure of the relative amounts of stars and dark matter.

Using a sample of 260 nearby elliptical galaxies from the ATLAS3D project, astronomers Michele Cappellari et al. constructed a two-dimensional map of stellar positions and motion. Through this study and an analysis of the particular types of stars (as determined by their spectra), the researchers were able to determine the number of stars in different mass ranges, something known as the initial mass function (IMF, not to be confused with the International Monetary Fund).

They determined that the IMF varies a lot from galaxy to galaxy, meaning that the formation of stars must depend on the detailed history of a galaxy's life—in contrast to earlier predictions.

Types of galaxies

Large galaxies are divided roughly into three categories: spiral, elliptical, and irregular, depending on the amount of structure and gas content. (Dwarf galaxies, which are less massive, fall into their own categories.) Following the convention established by Edwin Hubble, elliptical galaxies are known as early-type and spirals are known as late-type, since Hubble thought elliptical galaxies evolved into spirals. Over the decades, astronomers established that elliptical galaxies formed from merging smaller galaxies, so they aren't truly "early-type," but the name persists.

Galaxies provide the environment for star formation, gathering the gas and dust that are the raw ingredients, and shaping the clouds and the forces that drive their collapse. In the Milky Way, for example, the environment of star formation tended to produce a large number of small red stars, with relatively fewer yellow stars like our Sun, and far fewer massive blue stars. (The spectrum of a star—its color and the signature of the chemical composition of its atmosphere—provides information about its mass and where it is in its life cycle.)

Pinpointing individual stars in other galaxies is not always possible, especially since most stars are small. However, the spectrum of the entire galaxy reveals the stars that make it up. Since stars don't change mass over most of their lifetime, taking a statistical census reveals the relative number of stars in each mass range, which is the IMF. Until recently, it was uncertain whether every type of galaxy has the same IMF, or if elliptical galaxies and spiral galaxies might have different values.

Enter the ATLAS3D project. Taking a sample of 260 elliptical galaxies that are 42 million parsecs away or closer, Cappellari et al. generated maps of the distribution of stars in two dimensions, along with the statistical spectrum. (The third dimension—positions and motion of stars along the line of sight from the galaxy to Earth—is more difficult to obtain, but it's also not required if the primary goal is to determine the IMF.) They used this data to create several models for possible dark matter distributions, leading to a detailed picture of the structure of each galaxy.

In particular, the researchers determined the ratio of mass to light, or (M/L)stars, from the stars alone—that is, not including gas and dark matter. They found that elliptical galaxies with low (M/L)stars resembled spiral galaxies. But there were some with larger (M/L)stars values, clearly indicating that the "universal IMF" model doesn't hold.

This result held true generally for each type of dark matter distribution they tested, including a "control" model in which no dark matter was present. The independence of the conclusions on the amount of dark matter present means the result is independent of mass—there is no way to make all elliptical galaxies have the same IMF, much less galaxies of all types.

The results indicate that some galaxies are far more likely to be filled with higher mass stars than others, which implies that the birth of a star is influenced by the type of galaxy it is forming in.

It's even a bit more complicated than that, since observations have determined definitively that galaxies form hierarchically: large galaxies were built up by mergers between smaller galaxies. That means that some of the stars probably formed before the galaxy took on its present structure.

As Cappellari et al. argue, the difference in IMFs between high- and low-(M/L)stars means we must reexamine our assumptions about how stars form in the different galaxy types. Since stars cannot know in advance what type of galaxy they will eventually inhabit, elliptical galaxies must have formed stars in a different way than spirals, which depends on the particular history. Further observations should clarify exactly how this history determines the populations of stars we see today.